This invention relates to thermoplastic elastomer compositions with enhanced melt strength and reduced surface gloss for extrusion, injection molding, compression molding, calendering, thermoforming, blow molding, and foam processing, and articles made therefrom.
There is a need for recyclable materials that can be used as alternatives to polyvinyl chloride for the fabrication of articles. Polyvinyl chloride, often used with a plasticizer, can be formed into a rubbery, thin sheet for use as a skin layer over a rigid or soft substrate. Due to the combination of the tactile feel (softness) and the melt strength during processing, plasticized polyvinyl chloride can be a very desirable material. Polyvinyl chloride, however, is not easily recyclable or melt blendable with non-polar polymers, which has limited the utility of polyvinyl chloride to applications where recyclability is not desired. Materials with processing characteristics similar to polyvinyl chloride, such as high melt strength, are being actively sought.
A thermoplastic elastomer (TPE) is a material that exhibits rubber-like characteristics, yet may be melt processed with most thermoplastic processing equipment, such as by extrusion. The rubber-like characteristics typically desired are high extensibility, mechanical recovery, resiliency, and low temperature ductility. An olefinic thermoplastic elastomer includes primarily polymers manufactured by the polymerization of at least 50 mole percent olefinic monomers such as ethylene, propylene, butylene, iso-butylene, alpha-olefins, olefinic dienes, and the like. Olefinic thermoplastic elastomers including thermoplastic olefin blends, thermoplastic polymer alloy compositions, and dynamically vulcanized thermoplastic elastomers have been explored for such applications. Most olefinic TPE compositions, however, suffer from low melt strength, making such compositions undesirable for industrial processes where stretching and/or drawing of the material is required.
Some effort has been made in the art to overcome the melt strength deficiency of olefinic thermoplastic elastomers. One approach, as reflected in U.S. Pat. No. 4,365,044, is to blend polypropylene with a low density polyethylene, which has the desirable melt strength properties alone or together with other polymeric substances. Although the blend approach has met with some success, it is not preferred.
Another approach is to blend an irradiated partially crystalline polyolefin with high melt strength and a non-irradiated polyolefin, as disclosed in U.S. Pat. No. 5,508,318. This composition exhibits many desirable characteristics for extruded thin sheets, but suffers from a low melt strength at higher temperatures. The composition also has the disadvantage of higher cost due to the electron beam irradiation process and the subsequent number of melt blending steps required to achieve the desired material by incorporation of other raw materials and ingredients.
Likewise, there are a number of references that disclose the treatment of olefinic thermoplastic elastomers via a free radical generator. So-called dynamically vulcanized alloys (DVAs) can be prepared through the process of dynamic vulcanization such as that described in U.S. Pat. No. 3,806,558. Using this process, an elastomer can be crosslinked during melt mixing with a rigid thermoplastic polyolefin to yield a material that is melt processable, yet exhibit characteristics similar to thermoset elastomers. Compositions obtained with this process are micro-gel dispersions of cured elastomer is an uncured matrix of thermoplastic polymer. Commercial olefinic thermoplastic elastomer materials that use this technology of dynamic vulcanization are well known and are disclosed in U.S. Pat. Nos. 4,130,535 and 4,311,628. The materials disclosed in these patents are known as Santoprene(copyright) and utilize a phenolic resin to crosslink the olefin elastomer phase. The Santoprene(copyright) materials are melt processable and can be extruded into profiles such as sheets. They also tend to exhibit high melt strength, but have very little ductility and draw, which reduces the utility of the material technology for processing applications such as thermoforming, blow molding and forming.
The use of organic peroxide to crosslink the elastomer phase in an olefinic-based DVA is well known to those of ordinary skill in the art. For example, U.S. Pat. No. 3,758,643 discloses that peroxide 2,6-bis(t-butylperoxy)-2,5-dimethylhexane at a concentration of 0.05 to 0.4 weight percent is useful for crosslinking the elastomer phase in the olefinic DVA. The use of peroxide alone, however, can be detrimental to the high molecular weight polypropylene due to the beta-scission that occurs and results in a very low molecular weight for the thermoplastic phase. The consequences of this degradation include lower melt strength and poor solid-state mechanical properties.
U.S. Pat. No. 6,207,746 discloses a process for producing thermoplastic elastomers with olefin-elastomer and polypropylene via a radical-initiated mechanism. The patent further teaches that radical initiators above a concentration of 0.02 parts by weight of weight of the elastomer are required to accomplish a sufficient degree of crosslinking. It is known in the prior art that a combination of a free radical generators such as peroxide and the metal salt of an alpha, beta-unsaturated carboxylic acid may be used as a curing system for various polymers. U.S. Pat. No. 4,770,422 discloses a method for crosslinking polybutadiene by utilizing curative agents of about 25 to 40 parts by weight zinc (di)acrylate per 100 parts by weight polybutadiene and about 0.2 to 0.8 parts by weight per 100 parts of weight polybutadiene of peroxide. In addition, a method for curing compositions containing halogenated isomonoolefin/para-alkylstyrene random copolymers is disclosed in WO 98/12251. A polyvalent metal salt is present at a level of about 0.5 to 10 weight percent while the organic peroxide is present in a level of about 0.2 to 5 weight percent.
British Patent No. 1,091,818 discloses a method of curing a vulcanization mixture by adding an organic peroxide at a concentration of 0.1 to 10 parts by weight per 100 parts by polymer and a metal salt of methacrylic acid at a concentration of 1 to 10 parts by weight per 100 parts of polymer. The relatively high organic peroxide content disclosed therein would tend to cause significant chain scission of polypropylene, thereby leading to lower viscosity (or higher melt flow rate) and a resulting loss in melt strength properties.
Japanese Kokai Publication No. 7-33917 discloses a method for decreasing flow marks by breaking the propylene-containing component through heat degradation and then crosslinking the ethylene-containing components through the addition of an organic peroxide and an alpha, beta-unsaturated carboxylic acid metal salt. The ethylene content of the final blend is kept to 90-99.5 mole percent.
There remains a need for olefinic thermoplastic elastomers with improved melt strength and reduced gloss level, while still retaining other suitable characteristics such as ductility, for use in such industrial processes as extrusion, injection molding, compression molding, calendering, thermoforming, blow molding, and foam processing.
The invention relates to a modified olefinic thermoplastic elastomer composition formed by melt blending a thermoplastic olefinic elastomer with sufficient amounts of a free radical generator (A) and a co-curative agent (B) to promote crosslinking to a gel content of about 10 to 80%, wherein the olefinic thermoplastic elastomer and the modified olefinic thermoplastic elastomer composition each have a melt strength measured at a temperature of 220xc2x0 C. and a surface gloss level, with the melt strength of the modified olefinic thermoplastic elastomer composition divided by the melt strength of olefinic thermoplastic elastomer before addition of the curative agents provided a ratio of about 1.5/1 to 20/1, and with the surface gloss level of the olefinic thermoplastic elastomer composition after addition of the curative agents (GL2) divided by the surface gloss level of the olefinic thermoplastic elastomer composition before addition of the curative agents (GL1) providing a ratio which is equal to or greater than 0.01 but equal to or less than 0.5.
In a preferred embodiment, the ratio of the melt strength of the modified olefinic thermoplastic elastomer composition to the melt strength of the olefinic thermoplastic elastomer composition before addition of the curative agents is about 1.6 to 15. The sufficient amounts of components are typically about 0.001 to 0.05 pph of free radical generator (A) and about 0.001 to 7.5 pph of co-curative agent (B). Preferably, the free radical generator (A) is present in an amount of about 0.001 to 0.04 pph and the co-curative agent (B) is present in an amount of about 0.001 to 7 pph.
Typically, the free radical generator (A) includes one or more peroxides, persulfates, or diazo compounds, or mixtures thereof. Preferably, at least one organic peroxide is included. More preferably, the free radical generator (A) has a decomposition half-life of greater than about one hour at 120xc2x0 C.
The co-curative agent (B) typically includes metal salts of alpha, beta-unsaturated carboxylic acids, or alpha, beta-unsaturated carboxylic acids where the pending acid group has been neutralized, or mixtures thereof. In a preferred embodiment, the alpha, beta-unsaturated carboxylic acids or co-curative agent (B) includes acrylic, methacrylic maleic, fumaric, ethacrylic, vinyl-acrylic, itaconic, methyl itaconic, aconitic, methyl aconitic, crotonic, alpha-methylcrotonic, cinnamic, or 2,4-dihydroxy cinnamic acids, or mixtures thereof. In a more preferred embodiment, the alpha, beta-unsaturated carboxylic acids of co-curative agent (B) comprise acrylic, methacrylic, or maleic acids, or mixtures thereof. In yet another embodiment, the metals for forming the metal salts of co-curative agent (B) include zinc, lithium, calcium, magnesium, sodium, or aluminum, or mixtures thereof.
The blends of the invention can also include one or more thermal stabilizers, ultraviolet stabilizers, flame retardants, mineral fillers, extender or process oils, conductive fillers, nucleating agents, plasticizers, impact modifiers, colorants, mold release agents, lubricants, antistatic agents, and pigments.
The invention also relates to articles prepared by the method specified above, which are formed by thermoforming, blow molding, extrusion, injection molding, compression molding, calendering or foam processing.
The invention also encompasses each of these above-named embodiments with respect to the following two aspect of the invention.
First, the invention also relates to the modified olefinic thermoplastic elastomer compositions above formed by a process that includes melt blending a thermoplastic elastomer composition in the presence of sufficient amounts of a free radical generator (A) and a co-curative agent (B) to generate co-curative radicals to form a modified olefinic thermoplastic elastomer composition, wherein the ratio of the melt strength of the modified olefinic thermoplastic elastomer composition to the melt strength of the olefinic thermoplastic elastomer composition before addition of the curative agents is about 1.5 to 20, measured at a temperature of 220xc2x0 C., and the surface gloss level of the olefinic thermoplastic elastomer composition before addition of the curative agents (GL1) to the gloss level after addition of the curative agents (GL2) satisfies the formula: 0.01xe2x89xa6GL2/GL1xe2x89xa60.5.
Second, the invention also includes a method of forming a modified olefinic thermoplastic elastomer composition by melt blending a thermoplastic elastomer composition in the presence of sufficient amounts of a free radical generator (A) and a co-curative agent (B) to generate co-curative radicals to form a modified olefinic thermoplastic elastomer composition, wherein the ratio of the melt strength of the modified olefinic thermoplastic elastomer composition to the melt strength of the olefinic thermoplastic elastomer composition before addition of the curative agents is about 1.5 to 20, measured at a temperature of 220xc2x0 C., and the surface gloss level of the olefinic thermoplastic elastomer composition before addition of the curative agents (GL1) to the gloss level after addition of the curative agents (GL2) satisfies the formula: 0.01xe2x89xa6GL2/GL1xe2x89xa60.5.
In one embodiment, the method further includes forming the modified composition into an article. In a preferred embodiment, the forming includes extruding, thermoforming, injection molding, compression molding, blow molding, form processing or calendering the modified composition to form the article.